code: def test_unary_operations():
      OP_MAP = {"-": operator.neg, "+": operator.pos}
      for op, func in OP_MAP.items():
          z = func(datum.xxx)
          assert repr(z) == "({}datum.xxx)".format(op)
  
file_path: altair/tests/expr/test_expr.py
imports: import operator
  import pytest
  from altair import expr
  from altair import datum
  from altair import ExprRef
  from jsonschema.exceptions import ValidationError
label_desc: Use image node_id 1 for calling a global function with example usage: test_unary_operations() without return types